1. Field of the Invention
The present invention relates to a master cylinder which is used with a hydraulic brake system or a hydraulic clutch system for an automobile. More particularly, the invention relates to a master cylinder of the portless type having a piston provided with a valve means for establishing or interrupting communication between a pressure chamber and a fluid reservoir.
2. Description of Related Art
A typical master cylinder is disclosed, for example, in Japanese Patent Publication No. 56-142743 published on Nov. 7, 1981. That master cylinder includes a transmitting member for operating a valve means in accordance with the stroke of a piston which is connected to the transmitting member so as to move the transmitting member a distance in the axial direction. The transmitting member is engaged with a stopper ring and the stopper ring is snugly fitted in an inner bore of a cylinder body while the piston is at a rest position. The transmitting member removes a valve body from a valve seat so as to communicate a pressure chamber with a supply chamber to which brake fluid is supplied from the fluid reservoir. As a result of that construction, the axial length of the master cylinder may be reduced.
However, since the valve body which is made of an elastic material, such as rubber, is subject to deformation due to the hydraulic pressure in the pressure chamber, upon the return movement of the piston, the valve body cannot be removed from the valve seat. That means that fluid communication between the pressure chamber and the supply chamber is interrupted while the master cylinder is out of use and as a result a braking force is generated even though the master cylinder is not in use. In light of that problem, the distance between the valve body and the valve seat must be set at a sufficient distance so as to avoid the above mentioned brake application while the master cylinder is out of use. However, one of the problems associated with that construction is that during the movement of the valve body towards the valve seat, the pressure in the pressure chamber is not immediately increased in direct proportion to the depression of the brake pedal. Therefore, most of the pedal stroke is not directed towards increasing the pressure in the pressure chamber. That results in an uncomfortable or inconsistent brake operation.
Though this drawback can be overcome by providing an orifice at a communicating hole in the piston, that arrangement results in insufficient braking force since air mixed in the brake fluid cannot be returned to the supply chamber due to the very small movement of brake fluid between the supply chamber and the pressure chamber.